Release liners for pressure sensitive adhesive labels.
In the manufacture of pressure sensitive adhesive labels, particularly electronically imaged sheet labels, various types of siliconized release liners are used. Most of the siliconized release liners use a bleached super-calendered or clay-coated Kraft paper. For xe2x80x9cshelf markerxe2x80x9d or xe2x80x9cshelf talkerxe2x80x9d sheet labels, a polycoated paper tissue-back release liner is used.
The paper tissue-back liner provides the desired properties of roughness,coefficients of static and kinetic friction, blocking resistance and moisture barrier to enable the labels to be stacked and fed into a printer such as a laser printer. The coefficients of static and kinetic friction, which are partially a result of the roughness, primarily facilitate the transfer of a label through the printer. The blocking resistance, also partially a result of the roughness, primarily ensures that the liner-backed labels remain stacked without displacing themselves one to the other, while easily releasing themselves one from the other, when being removed from the stack and fed to the printer. The moisture barrier property of the paper tissue-back liner ensures that the liner-backed labels remain dimensionally stable. However, in an environment of high humidity, the paper tissue absorbs water vapor and the liner tends to upcurl. In an environment of low humidity the paper tissue disorbs water vapor and the liner tends to down curl.
There are several drawbacks to the use of the paper tissue-back liner. The machines for electronically printing labels cannot usually handle labels greater than 10.5 to 11 mils in thickness. The use of the paper tissue in combination with the other layers of the label, results in the label approaching the maximum design thickness that the printer can handle which is not desirable because of the potential for jamming the printer. The paper tissue in a non-laminated state is fragile and difficult to handle. In order to produce the tissue backed liner, a base paper is extrusion coated with a nonfilled polymer, typically LDPE. The tissue is laminated to the polymer at the extrusion nip, while the polymer is still molten. Once the polymer cools, the tissue is permanently bonded to the base paper. The non-coated side of the base paper is then extrusion coated with another polymeric layer, which polymeric layer is then silicone coated, usually in a separate operation.
Laminating the tissue to the polymer requires a separate unwind operation for the tissue, which increases waste and reduces productivity. Expertise is required in achieving the proper machine tension settings to laminate the tissue without wrinkles. Because the tissue is so fragile, breaks in the tissue are common, leading to machine down time and waste. Any defects in the tissue can lead to the entire liner being unacceptable. Additionally, the paper tissue itself is a relative costly component of the final label.
One object of the present invention is to replace the paper tissue-back release liner with a less expensive yet equally effective release liner.
Another object of the invention is to form the release liner on line.
Still another object of the invention is to provide a release liner which generally can be used as sheet liner or roll liner.
With the present invention, the prior art paper tissue is replaced by a filled polymer which provides several advantages. There is a reduction in the thickness of the final label because the single layer of filled polymer is thinner than the tissue paper/polymer composite. The filled polymer is less costly than the paper tissue and the filled polymer can be extrusion coated and processed on-line in the formation of a release liner.
The filled polymer has certain paper-like properties substantially similar to the properties of the paper tissue. The paper-like properties of the filled polymer are determined primarily by the properties of roughness, blocking resistance and coefficients of static and kinetic friction. The filled polymer also functions as a moisture barrier which dimensionally stabilizes the release liner and the final label. The filled polymer does not absorb and desorb water vapor in environments of high and low humidity and therefore, the release liner of the invention is not susceptible to curling.
Broadly the invention comprises a filled paper-like olefinic resin base layer extrusion coated onto one side of a paper substrate to form a synthetic paper-like laminate release liner base.
In a preferred embodiment of the invention, polyethylene extrusion resins are loaded with filler material in a range of 10-65% by weight, preferably 15 to 40% by weight based on the total weight of the final filled polyethylene resin.
In a particularly preferred embodiment, an olefinic extrudate is laminated to the other side of the paper substrate to form a release liner.
In another embodiment of the invention the release liner is characterized by a release film coated on the extrudate.
In still another embodiment of the invention, pressure sensitive adhesive is coated onto the release film.
In still another embodiment of the invention, a face stock is adhered to the pressure sensitive adhesive to form a label.
Also within the scope of the invention is the method of manufacture of the release liner in its various alternative embodiments as just described.